This invention relates to the optical coupling of optical fibres, generally single mode optical fibres, to injection lasers, and is particularly concerned with the achieving of high coupling efficiency and of long-term stability in the achieved coupling efficiency. Whilst it is generally advantageous to increase coupling efficiency, such an increase is generally achieved at the expense of an increased sensitivity of the coupling to the effects of malalignment, and this is liable to militate against stability of the coupling efficiency arising from stress-relief type movements provoked over the course of time for instance by thermal cycling. For many applications where high performance is required, the alignment between fibre and laser is required to be held constant to within about 0.11 .mu.m over a temperature range of -40.degree. to +85.degree. C., and through several hundred thermal shocks over the same temperature range.
Light from the laser may be coupled directly into the end of the fibre, in which case the fibre end may have a lensed refracting surface, or the light may be coupled via a separate discrete lens, typically a ball lens.
One type of coupling assembly has the injection laser mounted on a substrate provided with a platform in front of the output facet of the laser, to which platform the fibre is directly soldered once it has been moved into the alignment position found to provide optimal coupling efficiency. A serious drawback liable to occur with this alignment method is that the fibre position is disturbed in the course of the freezing of the solder pool by which it is secured to the platform. Such movement results from shrinkage effects that occur when the solder freezes, and generally the amount and direction of that movement is not predictable and reproducible. A resin may be used as an alternative to solder for the securing of the fibre to the platform, but this merely exchanges the movement problem engendered by the freezing of the solder with the movement problem engendered by the shrinkage occurring during the curing of the resin.
An alternative type of coupling assembly is described in GB 2 124 402 A to which attention is directed. In this assembly the fibre end is secured by solder in the bore of a length of hypodermic tubing, and then, once the fibre has been aligned, this tubing is secured to the platform by means of a saddle structure engaged around the tubing and laser beam welded to both the tubing and the platform. Movement of the fibre within the hypodermic tubing during the freezing of the solder does not disturb the final alignment of the fibre with respect to the laser because this alignment is not established until after such freezing of the solder has occurred. It is also to be noted that the heating involved in the laser beam welding is reasonably sharply localised, whereas if the fibre is secured in alignment by soldering, or by the thermally induced curing of a resin, the fibre is typically subjected to substantially higher temperatures while it is being so secured. Even so, it is found that the freezing of the weld-pool is liable to disturb significantly the alignment between fibre and laser. Using welding saddles and hypodermic tubes manufactured to a tolerance of .+-.10 .mu.m it has been found that the welding can result in displacement of up to 5 .mu.m. Such displacements can be corrected by subsequent straining of the hypodermic tube, but this amount of straining leaves residual stresses in the tube which are liable to relax over the course of time in the presence of thermal cycling and/or thermal shock.
Another mechanism by which misalignment may develop over the course of time between the fibre and the laser is associated with stress relief in the solder that secure the fibre in the bore of the hypodermic tubing. When a soft solder is used, it is found that over the course of time, in the presence of thermal cycling and/or thermal shock, relative movement is liable to occur between the fibre and hypodermic tube. Solder creep is found to occur, possibly occasioned in part by uneveness of plating within the bore of the tube as the result of imperfect cleanliness, by the presence of voids in the solder, and by the effects of differential thermal expansion, radial expansion of the solder being constrained by the encircling lube wall. It might be thought that such solder creep could be avoided merely by replacing the soft solder with a hard solder, but this has not been found to be an acceptable solution because the stresses involved in the hard soldering of an optical fibre within such a hypodermic tube have been found so great as upon occasion to induce spontaneous fracture of the fibre close to the point at which its lensed end protrudes from the end of the tube.